There are known so-called exhaust temperature sensors which measure the temperature of exhaust gas flowing through a path such as the inside of a catalytic converter or an exhaust pipe of automobile exhaust emission control device using a thermo-sensitive device.
A thermistor whose electric characteristics are sensitive to temperature is disposed inside a bottomed cylindrical metallic cover. In order to enhance the thermally sensitive response of the thermistor surrounded by the metallic cover, an insulating material which is good at thermal conductivity is disposed in a space formed between an inner circumferential surface of the metallic cover and an end surface of a sheath pin so that after received by the metallic cover, the heat of exhaust gas is transmitted to the thermistor through the insulating material. An electric signal produced by the thermistor whose electric characteristics are sensitive to temperature is transmitted through electrode wires to a control device in which the temperature is to be measured.
Such a temperature sensor is disclosed in patent documents 1 and 2. The temperature sensor, as illustrated in FIG. 9, includes a thermistor 501 serving as a temperature sensitive device, a sheath pin 505 having disposed therein signal lines 503 welded to a pair of electrode wires 502 joined to the thermistor 501, a temperature sensitive portion cover 504 that is a metallic cover disposed on a top end of the thermistor 501 to cover it, and a rib 601 retaining the sheath pin 505 at an outer circumference thereof. The installation of the temperature sensor in an exhaust pipe 800 is achieved by placing a holding member 602 and a nipple 701 on an outer circumference of the rib 601 and securing the nipple 701 to a boss 704 mounted in the exhaust pipe 800.
An internal combustion engine in connection with the exhaust pipe in which the temperature sensor is typically mounted usually vibrates during running thereof. Such vibration is transmitted from the exhaust pipe 800 to the sheath pin 505 of the temperature sensor through the boss 704 and the rib 601. Specifically, the rib 601 and the sheath pin 505 are welded together, so that the vibration is transmitted from the rib 601 directly to the sheath pin 505.
Consequently, the vibration of the sheath pin 505 becomes strong (high frequency and great amplitude), which may cause the temperature sensitive portion to vibrate at high acceleration.
An excessive stress may, thus be exerted on the top end of the sheath pin 505, the thermistor 505 disposed on the top of the sheath pin 505, or a welded portion between the sheath pin 505 and the rib 601.
The excessive stress on the thermistor 501 may result in breakage of the thermistor 501 or disconnection of the electrode wire 502 of the thermistor 505. The excessive stress on the weld between the sheath pin 505 and the rib 601 may result in cracks or breakage of the sheath pin 505.
Therefore, in FIG. 9, the length L4 between the top end of the sheath pin 505 held by the rib 601 and the top end of the temperature sensor is shortened greatly relative to the length L3 between the inner circumferential surface of the exhaust pipe 800 in which the temperature sensor is installed and the top end of the temperature sensor. This causes the resonance frequency of the top end of the temperature sensor to lie out of a resonance frequency band of the vibration of the exhaust pipe in which the temperature sensor is installed, thereby avoiding the disconnection of the signal lines 502, etc.
Patent Document 1: Japanese Patent First Publication No. 2002-350239
Patent Document 2: Japanese Patent First Publication No. 2006-47273
The structure of Patent document 1 or 2, however, face the problem that it is impossible to bring the resonance frequency of the top end of the temperature sensor out of the resonance frequency band of the vibration of the exhaust pipe, thus resulting in possibility of the breakage of the thermistor 501 or the disconnection of electrode wires 502.
When the rib 601 is prolonged toward the top end to increase the resonance (primary) frequency of the sheath pin 505, it will cause the resonance to be about 500 times, so that the stress acting on the electrode wires 502 located on the top end side of the temperature sensor to be increased greatly. This results in a difficulty in avoiding disconnection of electrode wires 502.
The present invention was made in order to solve the prior art problems. It is an object to provide a temperature sensor which reduces the transmission of vibration and is excellent in durability.
In order to achieve the above object, is a temperature sensor including a temperature sensitive device which is disposed in a flow path through which fluid flows and whose electric characteristic changes as a function of temperature of the fluid in the flow path, signal lines connected at top end sides thereof to said temperature sensitive device through electrode wires and at base end sides thereof to lead wires for connection with an external circuit, a sheath member retaining the signal lines therein, and a holding member which holds an outer circumferential surface of said sheath member directly or indirectly through another member, characterized in that a resonance (primary) frequency at a top end of the temperature sensor against acceleration in a radius direction of the temperature sensor is 480 Hz or less.
There was a conventional technical idea of increasing the resonance (primary) frequency at the top end of the temperature sensor to avoid the resonance arising from external vibration. However, the present invention was based on a technical idea opposite the conventional one. Specifically, the decrease in transmission of vibration to the top end of the temperature sensor is achieved by decreasing the resonance (primary) frequency at the top end of the temperature sensor down to 480 Hz or less, thereby avoiding the breakage of the thermistor 501 or disconnection of the electrode wires 502 even when the temperature sensor resonates.
The resonance (primary) frequency may also be 380 Hz or less against the acceleration in the radius direction of the temperature sensor.
This ensures the durability further and avoids the disconnection of the electrode wires for an increased period of time.
If a protruding length that is a distance between an inner circumference of said flow path and a top end of the temperature sensor on an axis of the temperature sensor is defined as L1, and a held length that is a distance between a top end of a portion of said sheath member which is held by the holding member directly or indirectly and the top end of the temperature sensor is defined as L2, then a relation of L1<L2 is preferably satisfied.
The protruding length L1 is changed frequently according to the object or intended purpose. For instance, the protruding length L1 is changed greatly between when the temperature of a central portion of the flow path is to be measured and when the temperature of an inner edge of the flow path is to be measured. Consequently, when L1<L2, the resonance (primary) frequency at the top end of the temperature sensor depends upon the protruding length L1 as long as a condition such as the diameter of the sheath member is constant. In other words, when the protruding length L1 is short, it will result in an increase in the held length L2, so that the resonance (primary) frequency at the top end of the temperature sensor will become great.
However, in the present exemplary embodiment, the held portion of the sheath member is designed to be located at the base end of the holding member, thereby permitting the held length L2 to be increased sufficiently even when the protruding length L1 is short, which results in a great decrease in resonance (primary) frequency at the top end of the temperature sensor regardless of the protruding length L1.
The breakage of the electrode wires and the temperature sensitive device located at the top end side of the temperature sensor caused by the resonance is, therefore, avoided.
If a diameter of a portion of the protruding length L1 which holds said temperature sensitive device is defined as a sensor outer diameter D, the sensor outer diameter D is 3.2 mm or less, and preferably the held length L2 is 75 mm or more.
This enables the resonance (primary) frequency at the top end of the temperature sensor to be decreased below 480 Hz as the sensor outer diameter D is decreased and the held length L2 is increased, thereby avoiding the disconnection of the electrode wires.
Preferably, the temperature sensitive device is disposed inside a metallic cover.
This shields the temperature sensitive device from the atmosphere of exhaust gas to avoid the reduction-caused deterioration of the temperature sensitive device.
Preferably, the temperature sensitive device is implemented by a thermistor.
This realizes the temperature sensor which is high in measurement accuracy.
Preferably, temperature sensitive device is embedded in a fixing member supplied inside a top end of said metallic cover.
This avoids collision of the temperature sensitive device with the metallic cover so that it is broken when the temperature sensor vibrates following external vibration. Further, the temperature sensitive device is secured by the fixing member inside the metallic cover, thus reducing the vibration of the temperature sensitive device caused by the resonance. This decreases the stress acting on the electrode wires which is developed by the resonance of the temperature sensitive device.
Preferably, the temperature sensitive device is sealed by glass.
In high-temperature environments, the metallic cover is oxidized, so that the concentration of oxygen within the metallic cover drops. It is, thus, necessary to avoid the reduction-caused deterioration arising from removal of oxygen from the temperature sensitive device. The reduction-caused deterioration of the temperature sensitive device is, therefore, avoided by sealing the temperature sensitive device using the glass. This ensures the stability in the measurement accuracy of the temperature sensitive device.